An electrorheological fluid is known as a fluid: (1) which is obtained, for example, by dispersing dispersed phase particles in an insulating dispersion medium; (2) whose character of viscosity changes from Newtonian viscosity to Bingham viscosity in accordance with an applied electric field; and (3) which generally shows a so-called Winslow's effect wherein the viscosity remarkably increases in response to the externally applied electric field so as to induce great shear stress. Since the Winslow's effect has a prompt response, the electrorheological fluid having this effect has been adapted to (A) a variety of driving devices such as a clutch, brake, engine mount, damper, valve, shock absorber, and actuator and (B) an ink jet of the electrorheological fluid.
It is known that an electrorheological fluid is obtained by dispersing cellulose, starch, silica gel, ion exchange resin, and sulfonated polymer particles possessing aromatic rings substituted with sulfonic acid groups, etc. in an insulating dispersion medium such as silicone oil, diphenyl chloride, or transformer oil. Moreover, the following fluids have recently been proposed: (1) a fluid wherein organic semiconductor particles such as poly(acene-quinone) are adopted as the dispersion phase (see the Japanese unexamined patent publication No. 61-216202/1986); (2) a fluid wherein dielectric substance particles are adopted as the dispersion phase, the dielectric substance having a three-layered structure wherein an electric conductive thin layer is provided on an organic solid particle and an insulating thin layer is further provided on the electric conductive thin layer (see Japanese unexamined patent publication No. 63-97694/1988); and (3) a fluid wherein a powder of an electric conductive composition is adopted as the dispersion phase, the electrically conductive composition being obtained by dispersing electric conductor particles such as carbon black in a resin (see Japanese unexamined patent publication No. 1-236291/1989).
The inventors of the present invention studied conventional closed devices for use with electrorheological fluids for the purpose of their practical use, and found the following problems. More specifically, in the conventional closed devices for use with electrorheological fluids, since an electrorheological fluid which is sealed inside the closed device contains some dissolved gases or bubbles, (1) a discharge occurs during operation of the closed device, thereby causing deterioration of the electrorheological fluid, and (2) the electrorheological fluid does not effectively operate in response to an applied electric field, thereby interrupting smooth operation of the closed device. Additionally, when the closed device is repeatedly operated, it sometimes occurs that the closed device is not well operated with repeatability, thereby further presenting the problem that the characteristics of the closed device deteriorate.